Growth regime map for liquid-bound granules: further development and experimental validation

An attempt was made to quantify the boundaries and validate the granule gro wth regime map for liquid-bound granules recently proposed by Iveson and Li tster (AlChE J. 44 (1998) 1510). This regime map postulates that the type o f granule growth behaviour is a function of only two dimensionless groups: the amount of granule deformation during collision (characterised by a Stok es deformation number, St(def)) and the maximum granule pore saturation, s( max). The results of experiments performed with a range of materials (glass ballotini, iron ore fines, copper chalcopyrite powder and a sodium sulphat e and cellulose mixture) using both drum and high shear mixer granulators w ere examined. The drum granulation results gave good agreement with the pro posed regime map. The boundary between crumb and steady growth occurs at St (def) of order 0.1 and the boundary between steady and induction growth occ urs at St(def) of order 0.001. The nucleation only boundary occurs at pore saturations that increase from 70% to 80% with decreasing St(def). However, the high shear mixer results all had St(def) numbers which were to o large. This is most likely to be because the chopper tip-speed is an over -estimate of the average impact velocity granules experience and possibly a lso due to the dynamic yield strength of the materials being significantly greater than the yield strengths measured at low strain rates. Hence, the m ap is only a useful tool for comparing the granulation behaviour of differe nt materials in the same device. Until we have a better understanding of th e flow patterns and impact velocities in granulators, it cannot be used to compare different types of equipment. Theoretical considerations also revea led that several of the regime boundaries are also functions of additional parameters not explicitly contained on the map, such as binder viscosity. ( C) 2001 Elsevier Science B.V. All rights reserved.